Impregnating a wood substrate with a fire retardant salt (or "salt") to obtain a fire-retarded wood substrate is known art. However, due to the water solubility and hygroscopicity of most known fire retardant salts, the utility of such a fire-retarded wood substrate is generally restricted to low (less than 50 percent) humidity applications. For example, if an ammonium phosphate-impregnated wood substrate is exposed to high (greater than 90 percent) humidity at ambient temperature, in approximately 3 days the fire retardant impregnant (salt) will leach therefrom. In other words, the salt will absorb sufficient water vapor to enable it to migrate to the wood substrate surface. Not only does this leaching deplete the salt content of the wood substrate, rendering it less fire resistant, but it also severely disfigures the wood substrate's surface, especially if finished. This leaching process is accelerated if the ammonium phosphate-impregnated wood substrate is exposed to running water, such as rainfall upon exterior woodwork.
Strother U.S. Pat. No. 3,565,679 teaches a composition comprising a cellulosic substrate impregnated with a polyalkylenepolyamine in combination with a condensation product of phosphorus pentoxide and ammonia. This composition demonstrates some leach resistance while utilizing the known fire retardant characteristics of ammonia and phosphorus. However, U.S. Pat. No. 3,565,679 compositions exposed to 95 percent relative humidity absorb sufficient water to cause some of the salt therein to migrate to the surface and thereat form small beads.
Lalk et al. U.S. Pat. No. 3,900,619 teach a process of inhibiting the staining of latex paint applied to a wood surface containing water-soluble tannin comprising applying to the wood substrate surface essentially the same water-soluble S-aryl cycloaliphatic sulfonium compounds here used. However, the Lalk et al. teaching confines its disclosure to the problem of tannin migration and surface treatments therefor, and does not address the dissimilar problem of inhibiting the leaching of a fire retardant salt from a wood substrate.